Insertion of CO2 into

The insertion of CO2 into one N-Si bond occurs quite readily under standard conditions when R, R = Me formation of l,4-bis(0-trimethylsilylcarboxyl)-l,4dihydropyrazine, however, requires more drastic measures (60°C, 50 bar CO2) than previously suggested [3]. The latter compound exists as a mixture of cis and trans isomers due to restricted rotation around the N-C(=0) bonds on the NMR time scale [4],... 

Scheme 4.4 Insertion of CO2 into epoxides and cleavage of cyclic carbonates. Step 1. Catalyst MgO, CaO. Step 2. Catalyst zeolites exchanged with alkah and/or earth metal ions.

Aluminum porphyrins first came to attention with the discovery that the simple alkyl complex Al(TPP)Et was capable of activating CO2 under atmospheric pressure. Both irradiation with visible light and addition of 1-methylimidazole were required for the reaction, which was proposed to proceed by initial coordination of the base to aluminum. The aluminum porphyrin containing direct product of CO2 insertion was not isolated, but was proposed on the basis of IR data to be (TPP)A10C(0)Et, which was then treated with HCl gas, presumably liberating propanoic acid, subsequently isolated as the butyl or methyl ester after reaction with 1-butanol or diazomethane, respectively [Eq. (5)]. Insertion of CO2 into the Al—C bond of an ethylaluminum phthalocyanine complex has also been reported. ... 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

The carboxylation reaction shown in reaction (11) is catalyzed by both nickel and palladium phosphine complexes. For example, Ni(dppe)Cl2 (where dppe is l,2-bis(diphenylphosphino)ethane) and Pd(PPh3)2Cl2 both catalyze reaction (11) [84-86]. Mechanistic studies have been carried out on these two systems, and the results indicate that two different mechanisms are involved. In the case of the Ni complex, the first step is the reduction of Ni(dppe)Cl2 to a transient Ni(dppe) species [85]. This process occurs in two one-electron steps (reaction 12). Bromobenzene then oxidatively adds to Ni(dppe) to form Ni(dppe)(Br)(Ph), reaction (13). The resulting Ni(II) aryl species is reduced in a one-electron process to form Ni(dppe)(Ph), which reacts rapidly with CO2 to form a Ni—CO2 intermediate as shown in reaction (14). The rate-determining step for the overall catalytic reaction is the insertion of CO2 into the Ni-aryl bond, reaction (15) step 1. This reaction is followed by a final one-electron reduction to regenerate Ni(dppe), the true catalyst in the cycle (reaction 15, step 2). 

Palladium(ll)/PliONa catalyst systems are also active in the reaction of butadiene and carbon dioxide, which again forms acids, lactones and esters (187). In the ittechanism of this reaction a hw(ij -allyl) species and a (n -allyl)(tj -atlyl) species are discussed. The insertion of CO2 into the t -allyl bond gives a carboxylate intermediate which liberates the acid.sand the lactones (Scheme 9). 

Carboxylate complexes are often synthesized by refluxing the acid with the metal salts (such as carbonate, sulfate, oxide, etc.), or by reaction of the sodium or silver salt of the acid with the metal halide. Insertion of CO2 into a-bonded organotransition metal species has also been used to generate carboxylate complexes, as has exchange reactions with metal alkoxides (see Section 3.3). 

Photogenerated cofactors can be employed to drive biocatalytic enzyme cascades including the photosynthetic carbon dioxide fixation process [184] (Figure 35). Photogenerated NADPH provides a two-electron relay for the insertion of CO2 into a-ketoglutaric acid (23) and pyruvic acid (21) in the presence of isocitrate dehydrogenase (IcitDH) and malic enzyme (MalE), respectively. In these photosystems, Ru(bpy)3 " acts as a photosensitizer, as a primary electron acceptor... 

Complex (23) also catalyzes reaction (147). In addition to the products (120)-(122), the diformate ester of 1,2-propyIene glycol was also formed. Other epoxides were studied and the complex [CoCl(PPh3)3] catalyzed the formation of the lactone (122) only. Formate complexes were believed to be present, resulting from the insertion of CO2 into a rhodium-hydride bond (equation 149). The formate complex then initiates the catalytic cycle. The general mechanism given in Scheme 54 was proposed for the reaction. 

In addition to the design of the solubility properties, the reactivity of organome-tallic species toward CO2 [13] (and many other potential supercritical reaction media) must be considered as important criteria for the choice of the catalyst. For example, the bisallyl ruthenium complex shown in Table 1 cannot be utilized as a precursor for ring-opening metathesis polymerization (ROMP) in SCCO2, because the insertion of CO2 into the Ru-allyl bond prevents the initiation mechanism [14]. Metal-mediated oxygen transfer to form CO and phosphine oxide was found to lead to deactivation of the [Ni(cod)2]/PMe3 (cod = 1,5-m-cycloocta-diene) catalyst system [15]. On the other hand, the reactivity of CO2 with metal... 

This reaction reflects the Lewis acidity of carbon dioxide in the presence of the basic R group. Insertion of CO2 into the metal-carbon bond of Grignard and alkylaluminum... 

A primary concern in the investigation of CO2 activation catalysis is an examination of the stoichiometric reactions this molecule undergoes with transition metal complexes. The most important of these reactions are the insertions of CO2 into metal-hydrogen, -carbon, and -oxygen bonds, because these often represent the first steps in the conversion of CO2 into organic compounds. 

Metal Hydrides. Insertion of CO2 into the metal-hydrogen bond of ci3-HM(C0)HL (M = W, Cr L = CO, PMe, P(0Me>3) has been found to be an extremely facile process (17-19). This is in contrast with the inability of CO2 to insert into the metal-hydrogen bond of the analogous neutral manganese hydrides. Although the group 6 and group 7 hydrides are isoelectronic, they have rather different properties. The hydride in HMn(CO)s is in fact rather acidic with a pKa of a 7. 

Metal Alkyls and Aryls. The insertion of CO2 into metal-carbon bonds allows for the formation of carbon-carbon bonds and is an important step in its activation. In an effort to further define the nature of the CO2 insertion process, we have examined its occurrence in the anionic group 6 complexes, ci -RM(C0) L (R = --CH3, -CeHs,... 

A backside attack of C02> on the alkyl carbon, analogous to the SO2 insertion mechanism was ruled out by studies of the a-carbon stereochemistry upon insertion (25). The insertion of CO2 into the metal carbon bond of threo-cis-W(CO) (L)(CHD-CHD-Ph) (L = CO and PMc3) proceeds with retention of configuration at the a-carbon (Scheme 1) (26). This is in contrast to the inversion of configuration at the alpha carbon found in backside SO2 insertion reactions. 

Because of the extreme CO lability of the W(C0)50R species, CO loss might be a prerequisite for CO2 insertion. However, the rate of CO2 insertion is not inhibited by the presence of carbon monoxide. Hence, we believe that an open coordination site is unnecessary for the insertion process to occur. The reaction is thought to involve a concerted insertion process, similar to that proposed and well documented for the insertion of CO2 into CH3W(C0)5. Nevertheless, insertion of CO2 into the W-OR bond is more facile than the corresponding process involving W-R. 

The reaction of transition metal hydrides and metal alkyls with CO2 frequently results in the formation of metal formates and carboxylates via an insertion of CO2 into a metal hydride or metal carbon bond. Step 2 of Scheme 1 (15-19). In some instances, the mechanism for this reaction has been investigated in detail. It has been found that the reaction can proceed by either a dissociative mechanism to produce a coordinatively unsaturated metal hydride as an intermediate, or it can occur by an associative mechanism (20-25). Thus, the metal hydride shown in Scheme 1 may or may not be required to be coordinatively unsaturated. Organometallic and metal phosphine complexes are again the two classes of complexes most commonly involved in CO2 insertions into metal hydrogen bonds (15-19). 

Although the reaction requires no external catalyst, carbon dioxide is activated by the interaction of its electrophilic carbon atom and the negatively polarized carbon in the ortho (or para) position of the phenolate ring. This mechanism is supported by the fact that even under high CO2 pressure no salicylic acid is formed from phenol. The product is stabilized via an a > y proton migration. The free acid is obtained from the sodium salt in reaction with an external proton (usually from sulfuric acid). Formally, this reaction can be regarded as the insertion of CO2 into an aromatic C-H bond however, the above mechanism disproves this idea. 

Takimoto and Mori reported that the nickel-catalyzed regio- and stereoselective ring-closing car-boxylation of the bis-1,3-diene 81 with dialkylzincs 82 gave the 3-allyl-4-alkenylpyrrolidines 83 in high yields (Scheme 29).108 The reaction proceeds through formation of the cyclic bis-jr-allylnickel complex 84, the insertion of CO2 into a nickel—carbon bond of 84, leading to 85, and the addition of RzZn to 85. 

Reversible carbon dioxide fixation. This complex undergoes reversible insertion of CO2 into the copper—carbon bond at ambient temperatures and ordinary pressure (equation I). ... 

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